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Author Topic: Dichroic v. Dichromatic (split from Re: Colour changing duck!)  (Read 2294 times)

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Offline krsilber

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Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« on: February 06, 2009, 10:56:49 PM »
That's definitely neodymium glass (same thing as Alexandrit] but definitely NOT dichroic. Your colour changing is caused by the glass absorbing different wavelengths of light. Dichroism is a light refraction property of crystals.

Here is a good website on neodymium glass BUT she is wrong when she uses the word dichroic; what she actually means is dichromatic.

There are two meanings of dichroic in optics.  One is that the color changes depending on the angle at which something is viewed because the substance splits light into multiple colors, which may be reflected or pass through the medium.  Coatings on dichroic glass beads, for instance, have this type of dichroism.  

The other is that the color changes depending on the polarity of light hitting it (fluorescent, incandescent, etc. lights have different polarities).  Neodymium glass belongs to this category.

Dichromatic is actually associated with the first type of dichroism, and doesn't apply to neodymium.

http://en.wikipedia.org/wiki/Dichroism
Kristi


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Offline Lustrousstone

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Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #1 on: February 07, 2009, 06:24:45 PM »
NO it's not dichroic Kristi, dichroism is to do with the polarisation of the light NOT particularly the wavelength. It's only the strength of the dichroism that depends on wavelength. Can I suggest reading the article again, in the nicest possible way of course  :fwr:

In neodymium glass, it's the wavelength of the light that matters not the polarity

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Offline Mike M

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Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #2 on: February 07, 2009, 11:24:42 PM »
Hi

This obviously this is an area where  terminology and defintions need to be exact

So please no one hit me over the head when I point out.....

'Alexandrit' is a copyright term still owned by Moser (who may have first developed this colour commecially) and refers to their glass only. -the usual term is Alexandrite -used by many other makers ranging from Rene Lalique, VSL, Murano and probably many others.

As for dating your ducky -very hard to say I'd guess 1970s and will probably be wrong!

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Offline krsilber

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Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #3 on: February 08, 2009, 12:00:02 AM »
Christine, which article are you referring to?  I'm not sure I see your point.


Wikipedia:
"Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are absorbed by different amounts."

Patent for neodymium-containing windshield glass:
"Glasses containing Neodymium Oxide experience “dichroism”. In artificial light, the Neodymium Oxide glass appears as a brilliant red. The color sensation not only varies with the type of illumination, but also with the thickness of the glass layer. In thin layers or with low concentrations of Neodymium Oxide these glasses are blue, in thick layers or with high concentration, red."
Kristi


"The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science."

- Albert Einstein

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Offline Lustrousstone

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Re: Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #4 on: February 08, 2009, 02:41:13 PM »
I was referring to the article you posted Kristi, which is all about dichroism - 1) materials/ devices splitting light (transmitting some wavelengths and reflecting others) and 2) an effect relating to the polarity of the light. You have to have light in different polarised states to achieve the second effect.

The colour changing properties of neodymium glass are simply due the wavelength of the light and the fact that neodymium oxide will absorb part of the spectrum. Your windshield patent is about absorption of yellow light, not about polarising light. It too uses the word dichroism incorrectly. Selective absorption of light by a particular material occurs because the selected frequency of the light wave matches the frequency at which electrons in the atoms of that material vibrate.

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Offline krsilber

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Re: Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #5 on: February 08, 2009, 06:48:06 PM »
I was referring to the article you posted Kristi, which is all about dichroism - 1) materials/ devices splitting light (transmitting some wavelengths and reflecting others) and 2) an effect relating to the polarity of the light. You have to have light in different polarised states to achieve the second effect.

Are you sure light from incandescent vs. fluorescence bulbs doesn't have different polarization (not polarity - different term; I used it wrong in my earlier post, too)?  Even things like reflection can change the polarization of light, and since light from the two sources is created in such different ways I don't see why it might not have different polarization.

The colour changing properties of neodymium glass are simply due the wavelength of the light and the fact that neodymium oxide will absorb part of the spectrum. Your windshield patent is about absorption of yellow light, not about polarising light.  It discusses different absorption in response to different light sources as well.  My understanding is not that dichroic materials polarize light, but that they look different dependent on the polarization of the light hitting them. It too uses the word dichroism incorrectly. Selective absorption of light by a particular material occurs because the selected frequency of the light wave matches the frequency at which electrons in the atoms of that material vibrate (is it possible to find a reference for this?  I thought absorption happened when the energy of the light was transferred to electrons, exciting them out of one orbit to the next one).

I'm the first to admit that I'm no expert on the physics of light.  But I don't understand why neodymium would reflect such obviously different wavelengths (i.e. colors) depending on whether the source is incandescent or fluorescent, both of which produce white light, even though they are somewhat different in their component spectra.  Do different fluorescent bulbs (e.g. cool white vs. warm white) result in different colors of neodymium glass?

What is your definition of dichromatic?  None of the definitions I've seen seem to apply to neodymium glass.
Kristi


"The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science."

- Albert Einstein

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Offline Lustrousstone

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Re: Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #6 on: February 08, 2009, 07:55:51 PM »
The component spectra is the point, neodymium absorbs in the yellow range, therefore what you've got left is the colour of the light transmitted. The glass is not reflecting or refracting; it is transmitting.

Light has to pass through something that "tidies" the waves to become polarised.  As transmitted, the lightwaves emitted from ordinary fluorescent or incandescent bulbs are all in different planes, i.e., completely unpolarised. See for a good explanation of light and polarisation

Same source for absorption

In its simplest form, dichromatic just means having two colours; neodymium glass is lilac under incandescent light and blue under fluorescent light. So if you are going to use any dichro... word, that's the one. I personally wouldn't use either, but to use dichroic is wrong.

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Offline krsilber

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Re: Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #7 on: February 09, 2009, 02:48:04 AM »
The article you refer to talks about a few different ways light can be polarized (or partially polarized).  For instance, "Polarization also occurs when light is scattered while traveling through a medium... This scattered light is partially polarized. Polarization by scattering is observed as light passes through our atmosphere."

There are a bunch of articles that talk about circular dichroism and neodymium, but most are on pay-per-view publishers' sites.

I don't know what the answer is, but I'm going to keep an open mind.
Kristi


"The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science."

- Albert Einstein

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Offline Frank

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Re: Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #8 on: February 09, 2009, 07:58:49 AM »
In terms of glass there are two completely different types that are called dichroic.

The original was a glass that shows a different colour in reflected or in transmitted light. This is caused by components in the glass, usually gold, that remain 'undisolved/suspended' (no time for term hunting) and has been around since Roman times.

In recent times the term has been applied to coatings that are used on glass, commonly in beads and modern stained glass sheet.

I am sure there was a big discussion about this a long time ago.

Fluorescent light is yellow/green and tungsten red/blue. Different colours of fluorescent do affect Neod. glass differently, as does the gamut of a chip in digital cameras resulting in effective trichromatism >:D

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Offline krsilber

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Re: Dichroic v. Dichromatic (split from Re: Colour changing duck!)
« Reply #9 on: March 25, 2009, 09:11:11 PM »
I found this while looking for info about the Lycurgus cup.  It's a slide show with some pretty heavy science and not much explanation, but I thought I'd add it here because it mentions metals and light polarization and frequency (slide 6).  Just another tidbit to add to the topic; maybe down the road it will help clarify something.
http://www.physics.usyd.edu.au/ugrad/tsp/talks/2005/LaurieField.pdf
Kristi


"The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science."

- Albert Einstein

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